Ion cyclotron resonance mass spectrometry and quadrupole ion trap mass spectrometry are based on the trapping and confining of ions in a localized volume of space using an electromagnetic field. In the case of ion cyclotron resonance, the trapping field is a combination of a static electric field and a magnetic field which creates the so-called Penning type trap. In the case of a quadrupole ion trap, the trapping field is an alternating electrical field and this trap is known as a Paul ion trap. In general the trapping action for both kinds of ion traps can be described as a pseudo-potential well for ions. If ions are formed inside of the trap, in the center of the potential well, then the trapping efficiency can theoretically be as high as 100% since ions do not have the energy to leave the potential well. However for practical purposes it is often beneficial to have an ion source externally located with respect to the trap when ions have to be injected into the trap. The externally produced ions have kinetic energy exceeding that necessary to escape the trap and therefore only a small number of the ions can be trapped resulting in low trapping efficiency.
External ion introduction has become a conventional and very powerful tool in ion trap mass spectrometry. The process of ion injection into an ion trap with high efficiency is crucial for the final sensitivity of the technique. In early ion injection attempts, off-axis ion ejection was suggested to improve trapping efficiency for the quadrupole radio frequency ion trap (O. C.-S., Schuessler, H. A. Confinement of Pulse-Injected External Ions in a Radio-Frequency Quadrupole Ion Trap, Int J. Mass Spectrom. Ion Phys.,1981, v 40, p.53-86). A similar approach was later introduced for ICR traps (P. Caravatti, U.S. Pat. No. 4,924,089). Off-axis ion injection results in a longer ion path inside the trap and a higher probability for an ion to lose energy as a result of a collision with a neutral gas molecule, and thus a higher trapping efficiency compared to the axial ion introduction.
For enhancing the trapping efficiency, a buffer gas pulsation was used to slow down ions inside the trap by means of collisions with buffer gas molecules. In this technique, one problem was the complexity of the vacuum system which used a pulsed valve for the buffer gas introduction (S. Beu et at., J.Am. Soc. Mass Spectrom. 1993, 4, 190-192). Another problem associated with this approach is the reproducibility of its results apparently due to problems with pressure control during buffer gas pulse. The pulsing of the trapping field has been used both for the ion cyclotron resonance (E. N. Nikolaev et at., Rapid Com. Mass. Spectrom., v.5, pages 260-262, 1991) and for quadrupole ion traps (U.S. Pat. Nos. 3,065,640 D. B. Langmuir et al, and 5,399,857 Doroshenko et at.,). These techniques work well for short bursts of ions, i.e. pulsed ion sources, but provide poor trapping efficiency for a continuous ion beam.
In a different approach to ion trapping, an additional radio-frequency field was superimposed on the main trapping field to improve ion trapping efficiency (A. Mordehai et al., Les Cahiers de Spectra, ISSN-0399-1172, No. 150, p.25, 1990). In this technique the additional radio-frequency electric field with mass specific frequency was utilized to increase trapping efficiency, however it was efficient only for a narrow range of mass-to-charge ratios.
All the above described ion trapping methods can be classified as active trapping techniques. There are several passive trapping techniques which are used when the ion trap is pressurized at a compromise pressure determined by adequate trapping efficiency and sensitivity while maintaining required mass resolution. For example, in the U.S. Pat. No. 5,268,572, Mordehai et al., the ion detector was placed in the differentially pumped vacuum chamber and the trap was pressurized up to 10.sup.-2 Torr to achieve efficient cumulative ion trapping. The standard ion trap scan technique (Kelley et al., U.S. Pat. No. 4,736,101) was used for ion detection. A trapping efficiency of about 10% has been reported in this method and only 50% of the trapped ions were actually detected (A. Mordehai et at., Rapid Comm. in Mass Spec., v.7, p.205-209, 1993).
There were also several different trap designs which were proposed to modify the electrical field in the trap during different modes of operation. A quadrupole ion trap with split end caps was developed to switch on and off the high order multipole fraction of the radio-frequency field (Franzen et al., U.S. Pat. No. 5,468,958). Several different ICR traps were suggested recently for specific applications (S. Guan et al., Int. J. Mass Spec. and Ion Proc., v. 146/147, 1995, p. 261). None of the previously developed traps were designed for optimum trapping of externally produced ions and thus provided low trapping efficiency with external ion sources.
In all prior art, trapping efficiency for cumulative trapping of externally produced ions of a wide mass range is extremely small (typically below 10%), thus resulting in poor overall sensitivity.